The presence of arsenic in waters and other aqueous solutions or streams may originate from or have been concentrated through geochemical reactions, mining and smelting operations, the land-filling of industrial wastes, the disposal of chemical agents, as well as the past manufacture and use of arsenic-containing pesticides. Because the presence of high levels of arsenic may have carcinogenic and other deleterious effects on living organisms and because humans are primarily exposed to arsenic through drinking water, the U.S. Environmental Protection Agency (EPA) and the World Health Organization have set the maximum contaminant level (MCL) for arsenic in drinking water at 10 parts per billion (ppb). As a result, federal and state governments and utility districts require a simple, low cost method for removing arsenic from ground water and other sources of potable water. In addition, those active in industries such as mining, metal refining, steel manufacturing, glass manufacturing, chemical and petro-chemical and power generation are looking to remove or reduce the amount of arsenic in their process streams, effluents and byproducts.
Arsenic can occur in the inorganic form in aquatic environments as the result of dissolution of solid phase arsenic such as arsenolite (As2O3), arsenic anhydride As2O5) and realgar (AsS2). Arsenic can have four oxidation or valence states in water, i.e., −3, 0, +3, and +5. Under normal conditions, arsenic is typically found in such solutions in the +3 and +5 oxidation states, usually in the form of arsenite (AsO2−1) and arsenate (As4−3). The oxidation state has a significant impact on the ability to remove the arsenic from solution. For example, effective removal of arsenic by coagulation techniques requires that the arsenic be in the arsenate form. Arsenite, in which the arsenic exists in the +3 oxidation state, is only partially removed by adsorption and coagulation techniques because its main form, arsenious acid (HAsO2), is a weak acid and remains un-ionized at pH levels between 5 and 8 at which adsorption is most effective.
Various technologies have been developed to remove arsenic from aqueous systems. Examples of such techniques include ion exchange with anion exchange resins, precipitation, electrodialysis, and adsorption on high surface area materials, such as alumina, activated carbon, various iron-containing compositions, lanthanum oxides and hydrous cerium dioxide. Some of these arsenic removal technologies have been made available commercially for point of entry systems, point of use and other small scale applications. However, such systems are focused on treating relatively small volumes of drinking water and are not suited or sufficiently robust for treating high volume industrial streams that can contain a diverse set of contaminants.
A simplified apparatus is needed that can be used to safely remove and dispose of arsenic from high volume aqueous streams and solutions that contain a diverse set of contaminants that may otherwise interfere with arsenic removal.